When is the AFP (alpha-fetoprotein) test typically measured during pregnancy?

The alpha-fetoprotein (AFP) test is typically measured during pregnancy to screen for certain fetal conditions, including neural tube defects and chromosomal abnormalities. The optimal timeframe for measuring AFP is between 16 to 18 weeks of gestation. This specific period is chosen because it coincides with the time when AFP levels produced by the fetal liver are most reliable for screening purposes. In early pregnancy, the levels of AFP are not yet at a measurable level or may not accurately reflect fetal conditions, which is why testing before 16 weeks is not standard practice.

Measuring AFP too late in the pregnancy, such as during the later weeks of gestation, can also yield less useful results for screening as the levels may change and may not accurately indicate potential issues. Therefore, the selection of this 16 to 18 week window is based on clinical guidelines and the physiological development of the fetus, making it crucial for early detection and intervention if needed.